Principles of Conventional Plant Breeding
Most of the commercial corn produced in the United States is produced from hybrid seed. The production of hybrid seed first requires the development of elite corn inbred lines that possess good combining ability to produce agronomically superior hybrids. The majority of hybrid seed produced in the United States is of the single cross type, wherein two inbred lines are intermated, or crossed, to produce what is termed seed of an F.sub.1 single cross hybrid. This seed is then sold to commercial grain growers who plant the seed and harvest the second generation, or F.sub.2 grain, for use on farm or for commercial sale.
The production of conventional single cross hybrid seed involves controlling the direction of pollination from one inbred to the other to assure the production of predominantly hybrid (cross pollinated) seed. Typically, directed pollination is accomplished by interplanting separate rows of female corn plants with male corn plants. The female corn plants that are male sterile may be produced by genetic mechanisms which render the corn tassel or pollen nonfunctional or by detasseling the plants in the field.
The development of corn hybrids requires the development of homozygous inbred lines, the crossing of these lines, and the evaluation of the crosses. Pedigree breeding, backcross conversion and recurrent selection breeding methods are used to develop the inbred lines from breeding populations. These breeding methods combine desirable traits from two or more inbred lines or various broad-based populations into breeding pools from which new inbred lines are developed by inbreeding or random mating and selection of desired phenotypes. The new inbreds are crossed with other inbreds and the resulting hybrids are evaluated to determine which have commercial value and agronomic usefulness.
The objective of typical plant breeding is to develop a hybrid with desirable traits such as resistance to diseases and insects, herbicide tolerance, tolerance to heat and drought, reduction of time to crop maturity, and improved agronomic quality. Because many crops are harvested mechanically, uniformity of plant characteristics such as germination time, stand establishment, growth rate, and fruit/seed size are also desirable.
The problem with conventional breeding techniques is that there are several grain quality traits, such as high oil concentration, that cannot readily be obtained in a high-yielding single cross hybrid. One solution to this problem has been proposed by Bergquist et al. in U.S. Pat. Nos. 5,704,160 and 5,706,603, incorporated herein by reference. A primary aspect of this method, known as the TOPCROSS.RTM. Grain Production System, is the interplanting of a pollinator corn plant possessing the characteristics for significantly increasing oil level in the resulting grain, with a male sterile hybrid corn plant. The resulting grain possesses an oil concentration much higher than would be expected for self- or cross- pollination of the fertile version of the hybrid corn plant.
In practice, the seed of the pollinator with improved grain quality traits is blended in small amounts with seed of an elite male sterile grain parent hybrid, but with sufficient pollinator seed to permit abundant pollen production for fertilization of the male sterile grain parent hybrid. The relatively low ratio of pollinator seed to male sterile grain parent seed (less than one pollinator plant to every three grain parent plants) takes advantage of the higher grain yield potential of the elite grain parent hybrid while assuring a sufficient population of pollinator plants to pollinate the male sterile grain parent plants.
Critical to the success of the TOPCROSS.RTM. Grain Production System is the use of a pollinator capable of enhancing the grain quality traits of the F.sub.1 grain. P725 was developed for this purpose. The present invention, when used as a pollinator, imparts high oil concentration to the resulting F.sub.1 grain without significant loss of yield.